Autofill puzzle and don't show it when there's only one puzzle

app/views/completions/_form.html.slim

@@ -21,11 +21,16 @@
       .form-floating
         = form.select :contestant_id, @contestants.map { |contestant| [contestant.name, contestant.id] }, {}, class: "form-select"
         = form.label :contestant_id
-  .row.mb-3
-    .col
-      .form-floating
-        = form.select :puzzle_id, @puzzles.map { |puzzle| ["#{puzzle.name} - #{puzzle.brand}", puzzle.id] }, {}, class: "form-select"
-        = form.label :puzzle_id
+  - if @puzzles.size > 1
+    .row.mb-3
+      .col
+        .form-floating
+          = form.select :puzzle_id, @puzzles.map { |puzzle| ["#{puzzle.name} - #{puzzle.brand}", puzzle.id] }, {}, class: "form-select"
+          = form.label :puzzle_id
+  - elsif @puzzles.size == 1
+    = form.hidden_field :puzzle_id, value: @puzzles.first.id
+  - else
+    = form.hidden_field :puzzle_id
   .row
     .col
       = form.submit submit_text, class: "btn btn-primary"